Price elasticity of demand

Price elasticity of demand (PED) measures the responsiveness of the quantity demanded of a good to a change in its own price. It is a crucial concept for firms in setting prices and for governments in levying taxes. The formula for calculation is the percentage change in quantity demanded divided by the percentage change in price. Due to the inverse relationship between price and quantity demanded (the law of demand), the result will always be negative. However, economists typically use the absolute value (ignoring the minus sign) for interpretation. This value allows us to classify demand into categories: elastic (PED > 1), inelastic (PED < 1), unitary elastic (PED = 1), perfectly inelastic (PED = 0), and perfectly elastic (PED = infinity).

Several factors determine whether the demand for a good is price elastic or inelastic. The most important is the availability of close substitutes; the more substitutes available, the more elastic the demand, as consumers can easily switch. The proportion of income spent on the good also matters; goods that constitute a large part of a consumer's budget, like a car, tend to have more elastic demand. The nature of the good is another key factor; necessities (e.g., bread, water) have inelastic demand, while luxuries (e.g., designer watches) have elastic demand. Addictive goods, such as cigarettes, tend to be very price inelastic. Finally, the time period considered is significant; demand becomes more elastic over time as consumers have more opportunity to find alternatives.

The gradient of a demand curve provides a visual clue to its elasticity, though they are not the same thing. A relatively steep demand curve indicates that demand is price inelastic, as a large change in price leads to only a small change in quantity demanded. Conversely, a relatively flat demand curve indicates price elastic demand. It is a common misconception that elasticity is constant along a straight-line demand curve. In fact, for a standard downward-sloping linear demand curve, PED varies along its length. Demand is price elastic on the upper part of the curve, becomes unitary elastic at the midpoint, and is price inelastic on the lower part. The two extreme cases are a perfectly inelastic curve (vertical line, PED=0) and a perfectly elastic curve (horizontal line, PED=infinity).

Understanding PED is vital for a firm's pricing strategy as it directly impacts total revenue (TR = Price x Quantity). When demand is price elastic (PED > 1), a decrease in price will lead to a proportionally larger increase in quantity demanded, causing total revenue to rise. If a firm with elastic demand raises its price, its total revenue will fall. In contrast, when demand is price inelastic (PED < 1), a firm can increase its price, and the resulting fall in quantity demanded will be proportionally smaller, leading to an increase in total revenue. If demand is unitary elastic (PED = 1), any change in price will be exactly offset by a proportional change in quantity, leaving total revenue unchanged.

Income elasticity of demand (YED) measures the responsiveness of the quantity demanded of a good to a change in consumers' income, with the price of the good unchanged. Because it is income rather than price that changes, YED describes a shift of the whole demand curve, not a movement along it. The sign of the answer is read first. A positive YED means a normal good: demand rises as income rises. A negative YED means an inferior good: demand falls as income rises, because consumers trade up to something they prefer. Among normal goods, a YED greater than 1 marks a luxury (demand grows faster than income) and a YED between 0 and 1 marks a necessity (demand grows, but more slowly than income). Firms use YED to forecast sales over the business cycle: sellers of luxuries gain most in a boom and lose most in a recession, while sellers of inferior goods can expect steadier or even rising sales when incomes fall.

Cross elasticity of demand (XED) measures the responsiveness of the quantity demanded of one good (A) to a change in the price of another good (B). It is the price of B that changes, so the demand curve for A shifts. Again the sign is read first. A positive XED means the goods are substitutes: when B becomes dearer, buyers switch to A, so demand for A rises. A negative XED means they are complements: they are used together, so dearer B means fewer of both are bought. An XED of zero means the goods are unrelated. The size of the value shows how close the relationship is: two brands of cola might have a large positive XED, while tea and coffee have a smaller one. Firms use XED to anticipate the effect of a rival's price change and to price complementary products, such as printers and ink cartridges, as a pair.